This rings a bell right now. :-)

Last week I integrated with a service using (shudder) SOAP.

I tried the XML call with curl and couldn't get it to work. I cursed, I frothed. The reply error was some Java dump, not helpful.

It took me embarrassingly long to realize I got the HTTP header fields for SOAP from something wrong I googled -- it worked when I copied the header line from old code I wrote before... :-) :-(